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  3. The Magic of Sound Music is one of the most beautiful forms of artistic expressions ever invented. In movies and plays, musi

The Magic of Sound Music is one of the most beautiful forms of artistic expressions ever invented. In movies and plays, musi

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问题                         The Magic of Sound
    Music is one of the most beautiful forms of artistic expressions ever invented. In movies and plays, music has an added function: it not only moves people but also can shock people.
    Our eardrums can withstand sound within 20 to 80 decibels(分贝). Once sound exceeds this limit, even beautiful music will become ear-splitting noise and harm health. A strong blast(响声)of high sound can twist and break a solid iron sheet. High sound of 150 decibels can kill a healthy rat.
    In movies, sometimes the hero can produce a sound that ordinary people can’ t hear and only those who have the same ability can feel. In nature, there is actually sound that is beyond our hearing. In physics, the sound that exceeds 20, 000Hz is called ultrasonic(超音波的). Dolphins, whales and bats can make such high-frequency sound. It does no harm to health.
    Sound less than 20Hz is called infrasonic(次声)waves. When we move, the air will vibrate. The vibration of air can produce infrasonic waves. As the frequency of infrasonic waves is close to that of people’ s internal organs, infrasonic wave may cause resonance(共振)in human bodies. As a result, people’ s vision may weaken and internal organs may rupture(断裂). However, whether an infrasonic wave can be used as a weapon depends on its intensity. If its intensity is very low, it won’ t damage internal organs or a person’ s health. If the intensity of infrasonic wave exceeds 160 decibels, it is extremely harmful. When wind blows at a force of 3 or 4 over the sea, it will produce infrasonic waves of several decibels. Only typhoons can produce infrasonic waves of over 100 decibels. At present, scientists can only produce infrasonic weapons in the lab with the help of advanced scientific tools and powerful electric power.
An ultrasonic sound______.
选项 A、is very loud
B、does harm to people’ s health
C、cannot be heard by people
D、is produced by the hero in movies
答案C
解析 第三段第二句表明，自然界中存在人类听不到的声音。根据第三段第三句可知，这种超过两万赫兹的声音在物理学中被称为超声波。由此可知，超声波是人类听不到的声音。
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